The gas phase fluidized bed process for polymerization permits a reduction in energy requirements and capital investment compared with more conventional processes. However, a limiting factor is the rate at which heat can be removed from an exothermic reaction reactor is limited by the need to prevent excessive entrainment of solids in the fluidizing gas stream as it exits for recycle from the reactor. Hence the amount of fluid which can be circulated and cooled per unit of time to remove the exothermic heat of polymerization is limited. As polymer product is produced and removed from the fluidized bed, reactants and catalyst material are continuously supplied either to the recycle stream or directly to the reaction zone of the fluidized bed.
The quantity of polymer exothermically produced in a given volume of the fluidized bed is related to the ability to remove heat from the reaction zone. Adequate heat removal is critical to maintain a uniform temperature within the fluidized bed and also to avoid catalyst degradation and polymer agglomeration. The temperature in the reaction zone is controlled below the fusing temperature of the polymer particles. The dew point of the recycle stream is the temperature at which liquid condensate begins to form in the recycle stream. By cooling the recycle stream below the dew point temperature and then injecting the two phase mixture thus formed into the reaction zone, the heat of vaporization of liquid is available to consume a portion of the exothermic heat of polymerization. This process is known as “condensed mode” operation of a gas phase polymerization process. As disclosed by J. M. Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790 and by M. L. DeChellis, et al. in U.S. Pat. No. 5,352,749, operation in “condensed mode” permits an increase in the space time yield of the reaction system—that is, an increase in the amount of polymer produced per unit of time in a given fluidized bed reactor volume.
Below the reaction zone of the fluidized bed is a gas distributor grid plate. Its function is to provide a uniform distribution of the recycle stream into the bottom of the bed. Below the gas distributor grid plate is located a bottom head mixing chamber where the recycle stream is returned after being compressed and cooled. As disclosed by S. J. Rhee, et al., in U.S. Pat. No. 4,933,149, flow deflection devices can be designed and positioned within the bottom head mixing chamber, to avoid excessive build up of entrained solids within the bottom head mixing chamber when operating without partial condensation of the recycle stream. When operating in “condensed mode”, a deflector geometry as disclosed in the '149 patent may be used to avoid excessive liquid flooding or frothing in the bottom head mixing chamber. However, as the condensing level is increased to further enhance heat removal and space time yield, excessive amounts of liquid can exist in the bottom head mixing chamber. This can lead to liquid pooling and instability problems.
The fluidized bed discharge process described by Aronson in U.S. Pat. No. 4,621,952 is an intermittent semi-batch process involving the transfer, by pressure differential, of solid and gas through multiple vessels. Being semi-batch in nature, the product removal capacity of a given facility is constrained by the time duration of the steps necessary to complete the process. The Aronson discharge process includes interconnecting conduits with valves between the vessels to permit gas venting and pressure equalization. The gas contains valuable raw materials for the fluidized bed reaction system. The gas may include unreacted monomers and comonomers; inert materials are also common. Aronson discloses that the discharge process obtains the desired transfer of solid material while minimizing gas losses. Aronson does not, however, monitor liquid in the product discharge tanks or inject fluid to a point higher than product withdrawal.
As disclosed by Jenkins, et al., in U.S. Pat. No. 4,543,399 and by Aronson in U.S. Pat. No. 4,621,952 the polymer product is intermittently withdrawn from the fluidized bed at an elevation above the gas distributor grid plate. At increasing levels of partial condensation of the recycle stream the likelihood increases that undesirably high levels of liquid phase may exist in lower portions of the fluidized bed. Unfortunately during a product discharge event liquid can be carried out of the reactor along with the granular polymer and gases. Because of the depressurization which takes place during product discharge, the liquid expands and vaporizes, which may cause temperature reduction and pressure elevation within the discharge equipment. This can reduce the fill efficiency of the discharge system, and the reduction in fill efficiency in turn reduces the production capacity by increasing the time to depressurize, and increases the raw material usage of the process. Accordingly, it has been difficult to increase the liquid content in the recycle stream to enhance the efficiency of removing the heat of reaction.
In Chinh et al, in U.S. Pat. No. 5,804,677, the patentees assert they describe the separation of liquid from a recycle stream; the separated, collected liquid is injected into the fluidized bed above the gas distributor plate. The present invention also injects recycle liquid above the distribution plate, but applicants' liquid is handled as a liquid/gas mixture and as a more or less predetermined fraction of the recycle stream, as a slip stream, divided simply and directly in the recycle conduit. Because of the applicants' manner of separating, we are able to enhance the ratio of liquid to gas in the slip stream as compared to the withdrawn recycle stream, and thus simply and directly, without additional or special equipment, improve heat exchange efficiency and enhance the space/time yield of the process. In addition, we are able to optimize the product discharge cycle by coordinating liquid volume in the discharge tanks with the rate of injection of liquid above the distributor plate.